1. Field of the Disclosure
The disclosure relates to ultra-high power fiber laser systems. Particularly, the disclosure relates to an ultra-high power multimode (“MM”) fiber system emitting tens of kWs, MM laser output and configured with a mechanism which is operative to efficiently filter out unwanted forward-propagating and backreflected cladding light.
2. Prior Art
Multi kW fiber laser systems are used in a growing number of applications. As fiber lasers mature towards commercial deployment, an intense focus on their power, quality, reliability and that of their components is required. To enable the desired power levels, multiple low-mode (“LM”) fiber laser systems are optically and mechanically coupled together in a LM-MM combiner. To efficiently perform at high kW power levels, the combiner should successfully deal with a few structural difficulties some of which, such as mechanical coupling of fibers and power loses in forward and backreflection directions, are of particular interest here.
In general, a process of fabricating high power combiner includes fusing aligned output fibers of respective fiber lasers/amplifiers in a bundle, tapering the latter, cleaving and splicing the tapered bundle to a system output delivery fiber. The fabrication of the combiner which initially assumes a bow-tie configuration, whose waist is farther severed in half, may result in structural defects (burrs) of the outer cladding which further, during the combiner's deployment, may detrimentally affect the quality of the output laser beam and power thereof.
With outputs of fiber laser systems reaching tens of kWs, both forward and backreflected propagating core-guided light tends to bleed into the adjacent cladding at air-quartz interfaces and splices between various fibers as it propagates through the system. Once in the cladding, high power signal light induces thermal loads on a polymeric coating surrounding the cladding. The coating is configured to minimize the structural damages that a fiber may experience as a result of external mechanical loads applied to the fiber. Both forward-propagating signal light and, particularly, backreflected light—light reflected from the surface to be laser treated—may bled into a cladding which is damaging both to a combiner itself and to system components upstream therefrom which are particularly vulnerable when they are exposed to the backreflected light. Accordingly, both forward and backward propagating cladding light have be removed from a waveguide.
In summary, to attain an optical power of up to several tens of kWs at the output of the LM waveguide, the combiner needs a special configuration which would allow the following:                (1) Reliably fix input fibers together without degrading the quality of the output beam and loosing light power;        (2) Efficiently distribute and utilize power losses of forward and backward propagating light; and        (3) Provide protection of the fibers from environmental impurities as a result of heat-induced deformation.        
A need therefore exists for an ultra-high power MM fiber laser system meeting the above-articulated conditions.